Pinned furnace tubes

ABSTRACT

In an embodiment of the invention, furnace tubes for cracking hydrocarbons having a longitudinal array of pins having i) a maximum height from 0.5-1.3 cm; ii) a contact surface with the tube, having an area from 0.1%-10% of the tube external surface area iii) a uniform cross section along the length of the pin. (i.e. they are not tapered); and iv) a length to diameter ratio from 1.5:1 to 0.5:1 have an improved heat transfer over bare fins and reduced stress relative to a fined tube.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/612,509 filed on Feb. 3, 2015 entitled “Pinned Furnace Tubes” whichis herein incorporated by reference in its entirety.

FIELD

In some embodiments, the present invention relates to the field ofcracking paraffins to olefins and, for example, to pins (spines orstuds) in a longitudinal array on the external surface of the processcoil(s) in the radiant section of a cracking furnace. The pins can bespaced apart in a regular pattern, or the spacing and length of the pinsmay vary to provide a profile to the array of pins. The profile of thearray may be varied depending on the exposure of the coil to localradiation intensity in a furnace. A size of a single pin should be suchthat its base (a contact surface with a coil) should not exceed 10% ofthe coil external surface area, and its height should not exceed 20% ofthe coil internal diameter.

In some embodiments, these pins increase net transfer of radiant andconvective heat from flame, combustion gases and surrounding furnacewalls, into the external surface of the process coil.

BACKGROUND

The field of heat exchanger designs is replete with applications of finsto improve the heat transfer. In some embodiments, this is heat transferby forced convection mechanism. The heat transfer by forced convectiontakes place between a solid surface and fluid in motion, which may begas or liquid, and it comprises the combined effects of conduction andfluid flow. This type of heat transfer occurs in most of theconventional heating systems, either hot water or electric andindustrial heat exchangers.

In the cracking process of a paraffin, such as ethane or naphtha, feedflows through a furnace coil (pipe) that is heated up to 1050° C. insidethe radiant section of a cracking furnace. At these temperatures, thefeed undergoes a number of reactions, including a free radicaldecomposition (cracking), reformation of a new unsaturated product andthe coproduction of hydrogen. These reactions occur over a very shortperiod of time that corresponds to the feed residence time in a coil.

The interior of the radiant section of the furnace is lined with heatabsorbing/radiating refractory and is heated, for example, by gas firedburners. The heat transfer within the furnace, between flame, combustiongases, refractory and the process coils is mostly by radiation, and alsoby forced convection.

There is a drive to improve the efficiency of cracking furnaces as thisreduces process costs and greenhouse gas emissions. There have been twomain approaches to improving efficiency; improving heat transfer to thefurnace coils, i.e., from flame, combustion gases and refractory wallsto the external surface of a process coil, and improving heat transferwithin the coil, i.e., from the coil walls into the feed flowing insidethe coil.

One of the methods representing the second approach, is the addition ofinternal fins to the inner walls of the furnace coil, to promote the“swirling” or mixing of the feed within the coil. This improves theconvective heat transfer from the coil walls to the feed as theturbulence of the feed flow is increased and the heat transferringsurface of the hot inner wall of the coil is increased as well.

U.S. Pat. No. 5,950,718 issued Sep. 14, 1999 to Sugitani et al.,assigned to Kubota Corporation provides one example of this type oftechnology.

The papers “Three dimensional coupled simulation of furnaces and reactortubes for the thermal cracking of hydrocarbons”, by T. Detemmerman, G.F. Froment, (Universiteit Gent, Krijgslaan 281, b9000 Gent—Belgium,mars-avri, 1998); and “Three dimensional simulation of high internallyfinned cracking coils for olefins production severity”, by Jjo deSaegher, T. Detemmerman, G. F. Froment, (Universiteit Gent1,Laboratorium voor Petrochernische Techniek, Krijgslaan 281, b-9000 Gent,Belgium, 1998 provide a theoretical simulation of a cracking process ina coil which is internally finned with helicoidal and longitudinal fins(or, rather, ridges or bumps). The simulation results are verified bylab scale experiments, where hot air flows through such internallyfinned tubes. The papers conclude that the tube with internal helicoidalfins performs better then with internal longitudinal fins and that theresults for “a tube with internal helicoidal fins are in excellentagreement with industrial observations”. However, no experimental dataare provided to support these conclusions. There is also no comparisonmade to the performance of a bare tube, with no internal ribs or fins.The authors agree that one potential disadvantage of such coils withinternal fins is that carbon deposits may build up on the fins,increasing the pressure drop through the tube.

U.S. Pat. No. 3,476,180 issued Nov. 4, 1969 to Straight Jr. et al.,assigned to Esso Research and Engineering Company teaches tubes for usein the convection section of a cracking furnace. There are pins that areon the surface of the downward face of the tubes in the convectionsection of the furnace. The pins are tightly packed and there is nodimension given for the length of the pin. In the convection section ofthe furnace, the feed is relatively cool. The heat loss from the pins islow. In the radiant section of the furnace, it may be necessary to limitthe length of the pin or the pin may become a radiator, in effect,dissipating heat from the tube. The patent fails to suggest the subjectmatter of the present claims.

U.S. Pat. No. 5,437,247 issued Aug. 1, 1995 to Dubil et al., assigned toExxon Research and Engineering Company teaches elongated platespivotably mounted on at least one horizontal tube in a vertical row thatis one row removed from the wall. When the plate is pivoted down, itprevents channeling of the hot gases through the convection section ofthe furnace (flue).

Canadian Patent No. 1,309,841 issued Aug. 25, 1988 to Fernandez-Baujinet al., assigned to Lummus Crest Inc., USA teaches putting “studs”(“pins”) on the external and internal surfaces of pyrolysis coils usedin the radiant section of a cracker. The “pins” are not arranged inlongitudinal rows. Additionally, the “pins” have a length from 0.5 to0.75 inches. This is longer than the pin lengths disclosed herein.

U.S. Pat. No. 6,250,340 issued Jun. 26, 2001 to Jones et al., assignedto Doncasters PLC, teaches pipes for chemical reactions, such as,furnace tubes having internal grooves

The report High Efficiency, Ultra low emissions, Integrated ProcessHeater by TIAX LLC of Jun. 19, 2006 to the U.S. Department of Energy,Golden Field Office discloses furnace tubes having studs 2 inches longand 0.5 inches in diameter. The studs were placed in longitudinal arrayson the side of the furnace tube facing the refractory wall. The studshave a length of 2 inches (page 3-26). This teaches away from thesubject matter disclosed herein.

In some embodiments, the present invention seeks to provide an enhancedheat transfer, comparable to that of a fin, while reducing the stress onthe tube or pipe.

SUMMARY

Disclosed herein is a tube for use in the radiant section of a furnacefor cracking hydrocarbons to produce olefins having on its exteriorsurface a series of pins in one or more linear arrays parallel to thelongitudinal axis of the tube, said pins having:

-   -   i) a height from 0.5 to 1.3 cm (0.25 inches to 0.5 inches)        (e.g., from 3% to 15% of the tube outer diameter of the tube);    -   ii) a contact surface with the tube, having an area from 0.1% to        10% of the tube external surface area;    -   iii) a uniform cross section along the length of the pin. (i.e.,        they are not tapered)    -   iv) length to diameter ratio from 1.5:1 to 0.5:1

Also provided is the above tube, wherein the distance betweenconsecutive pins within a given linear array is from 0.1 to 5 times thediameter of the pin.

Also provided is the above tube, comprising from about 55 to 65 weight %of Ni; from about 20 to 10 weight % of Cr; from about 20 to 10 weight %of Co; and from about 5 to 9 weight % of Fe and the balance one or moreof the trace elements.

Also provided is the above tube, further comprising from 0.2 up to 3weight % of Mn; from 0.3 to 2 weight % of Si; less than 5 weight % oftitanium, niobium and all other trace metals; and carbon in an amount ofless than 0.75 weight % the sum of the components adding up to 100weight %.

Also provided is the above tube, comprising from 40 to 65 weight % ofCo; from 15 to 20 weight % of Cr; from 20 to 13 weight % of Ni; lessthan 4 weight % of Fe and the balance of one or more trace elements andup to 20 weight % of W the sum of the components adding up to 100 weight%.

Also provided is the above tube, further comprising from 0.2 up to 3weight % of Mn; from 0.3 to 2 weight % of Si; less than 5 weight % oftitanium, niobium and all other trace metals; and carbon in an amount ofless than 0.75 weight %.

Also provided is the above tube, comprising from 20 to 38 weight % ofchromium from 25 to 48, weight % of Ni.

Also provided is the above tube, further comprising from 0.2 up to 3weight % of Mn, from 0.3 to 2 weight % of Si; less than 5 weight % oftitanium, niobium and all other trace metals; and carbon in an amount ofless than 0.75 weight % and the balance substantially iron.

Also provided is the above tube, wherein the cross section of the pin isround.

Also provided is the above tube, wherein the cross section of the pin isquadrilateral.

Also provided is the above tube, wherein the pins in a linear array areof uniform height.

Also provided is the above tube, wherein the spacing between pins in alinear array is from 1 to 3 times the diameter of the pin.

Also provided is the above tube, wherein the pins in a linear array areof different heights to provide a profile to the array.

Also provided is the above tube, wherein at least part of the profile ofan array is a taper or curve.

Also provided is the above tube wherein the central axis of the pin isat an angle from 90° to 60° relative to the external surface of thetube.

Disclosed herein are methods for making a tube as described above bywelding (for example, electrical) any stud shaped piece or strip to thesurface of the tube and then cutting the stud shaped pieces at a desiredlength.

DETAILED DESCRIPTION

Other than in the operating examples or where otherwise indicated, allnumbers or expressions referring to quantities of ingredients, reactionconditions, etc. used in the specification and claims are to beunderstood as modified in all instances by the term “about.”Accordingly, unless indicated to the contrary, the numerical parametersset forth in the following specification and attached claims areapproximations that can vary depending upon the properties desired. Atthe very least, and not as an attempt to limit the application of thedoctrine of equivalents to the scope of the claims, each numericalparameter should at least be construed in light of the number ofreported significant digits and by applying ordinary roundingtechniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical values, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of “1 to 10” is intended to include all sub-ranges between andincluding the recited minimum value of 1 and the recited maximum valueof 10; that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10. Because the disclosednumerical ranges are continuous, they include every value between theminimum and maximum values. Unless expressly indicated otherwise, thevarious numerical ranges specified in this application areapproximations.

All compositional ranges expressed herein are limited in total to and donot exceed 100 percent (volume percent or weight percent) in practice.Where multiple components can be present in a composition, the sum ofthe maximum amounts of each component can exceed 100 percent, with theunderstanding that, and as those skilled in the art readily understand,that the amounts of the components actually used will conform to themaximum of 100 percent.

In the manufacture of ethylene, a feed, (for example, a feed selectedfrom C₂₋₄ alkanes), and naphtha is fed into one or more furnace coilscomprising straight tubes and “U” bends which pass through a crackingfurnace. The furnace, schematically shown in FIG. 1, includes two mainparts: the convection section (1) where the feed (2) is initiallypreheated and initial cracking may occur, and the radiant section (3)where most of the final cracking process takes place. The radiantsection of the furnace comprises the inlet (4), located downstream ofthe convection section (1) which accounts for about half of the furnaceradiant section and is sometimes referred to as a “cold” box, and theoutlet (5) referred to as “hot” box. The feed flows through the processcoil (6) which includes a long coil (7), suspended inside the inlet (4)and outlet (5) radiant sections of the furnace. To increase the lengthof the coil and, thus, to allow for the adequate residence time of thefeed inside both radiant parts of the furnace, the coil comprisesmultiple vertical straight tubes (7), referred to as “passes”,inter-connected by return bends (8) (U-shaped elbows). As the feed flowsthrough the coil passes in the inlet radiant furnace section (4), it isheated approximately to the temperature at which cracking reactionsbegin and further cracking occurs. Next, the feed leaves the inletradiant section and flows through the passes of the process coils in thefurnace outlet radiant section (5). In this section, the feed is furtherheated, the cracking reaction is progressing, until the final productsexit the coil (9) and are further treated (e.g., quenching andseparation) and recovered downstream. In both parts of the furnaceradiant section, the feed is heated by flames and by combustion gasesgenerated by the burners (10) which are mounted on the furnace walls andon the furnace bottom. Heat transfer from combustion gases and flames tothe processing coil (6) occurs predominantly by radiation and also, to alesser extent, by the mechanism of forced convection. Flame andcombustion gases heat not only the coils but also the furnace walls. Thewalls which are lined with a heat absorbing/radiating refractory,radiate heat on the coil, thus contributing to heating process of theflowing feed (2) as well.

If the efficiency of the heat transfer to the process coil in a furnaceradiant section is maximized or just increased, fuel consumption by theburners can be reduced and, consequently, so are greenhouse gasemissions reduced. The increased efficiency of heat transfer in theradiant section provides also another possibility, such that the fuelconsumption can be kept unchanged, but furnace capacity can beincreased, i.e., higher feed flow rates can be cracked in the coil.

In an embodiment of the present invention, at least a portion of theexternal surface of one or more passes of the coil or furnace tube isaugmented with relatively small pins or studs in a longitudinal array(e.g., along the major axis of the tube.)

The pins or studs may have any cross section, such as, a quadrilateral(e.g., rectangular or square) or round or oval. The pin or stud willhave a height (length) from about 3% to 15% of the outer diameter of thetube, or, for example, from 0.5 cm to 1.3 cm, or, for example, from 0.5cm to 1.2 cm, or, for example, from 0.8 cm to 1 cm. The base of the pinmay cover from 0.1 to 10%, or, for example, from 1 to 8%, or, forexample, from 2 to 5% of the external surface area of the pipe or tubedefined by the sum total contact area of all pins divided by the totalsurface area of the tube.

${{area}\mspace{14mu}{ratio}\mspace{14mu}( {{meter}\mspace{14mu}{length}} )} = {\frac{{Total}\mspace{11mu}{pin}\mspace{14mu}{base}\mspace{14mu}{area}}{{Total}\mspace{14mu}{tube}\mspace{14mu}{extract}\mspace{14mu}{area}} = \frac{\#\frac{pins}{m} \times \frac{\pi\;{OD}_{pin}^{2}}{4}}{\pi\;{OD}_{tube} \times 1m}}$

The length to diameter ratio of the pin may be from 1.5:1 to 0.5:1, forexample, from 1:1 to 0.5:1. In a longitudinal array the spacing of thepins may be from 5D (diameter of the pin) to D/10, for example, from0.5D to 5D, or, for example, from 1D to 3D. However, it should be noted,that in any array, the spacing of the pins need not be uniform. Forexample, the spacing could be wider at the middle of the tube and closertowards the end of the tube. What is more desirable is to increase theheat flux into the tube. While generally it is easiest if the centralaxis of the pins are perpendicular to the surface of the tube they maybe at an angle from 90° to about 60° relative to the surface of thetube. The pins or studs may have a uniform cross section along theirlength and are not tapered.

The longitudinal arrays are to be radially spaced apart along thesurface of the tube by an angle from 30° to 180° so there may be from 12to 2 longitudinal arrays on a pipe, for example, from 2 to 6longitudinal arrays are used (e.g., radially spaced from 180° to 60°).The arrays need not be circularly parallel. That is, adjacent arrayscould be offset so that the pins in one array match spaces in anadjacent array. Additionally, an array need not be uniform in heightalong its entire length. The array could have one or more sections ofreduced height. For example, the array could reduce from a maximumheight in the middle to a minimum height at each end (an inward facingparabola) or vice versa (an outward facing parabola) so that the arrayhas a profile.

The location and arrangement of the arrays of pins should maximizeradiant and convective heat flux into the coil. The location of thearrays and spacing and heights of the pins need not be uniform.

In designing the pins, care should be taken so that they absorb moreradiant energy than they may radiate. This may be restated as thetransfer of heat through the base of the pin into the coil should exceedthat transferred to the equivalent surface on a bare coil at the sameoperational conditions. If the concentrations of the pins becomeexcessive and if their geometry (height and diameter) is not selectedproperly, they may start to reduce heat transfer, due to thermal effectsof excessive conductive resistance, which is not desirable. The properlydesigned and manufactured pins will increase net radiation andconvective heat transferred to a coil from surrounding flowingcombustion gasses, flame and furnace refractory. Their positive impacton radiation heat transfer is not only because more heat can be absorbedthrough the increased coil external surface so the contact area betweencombustion gases and coil is increased, but also because the relativeheat loss through the radiating coil surface is reduced, as the coilsurface is not smooth any more. Accordingly, as a pin radiates energy toits surroundings, part of this energy is delivered to and captured byother pins, thus it is re-directed back to the coil surface. The pinswill also increase the convective heat transfer to a coil, due toincrease in coil external surface that is in contact with flowingcombustion gas, but also by increasing turbulence along the coil surfaceand by reducing the thickness of a boundary layer.

The pins may comprise up to 5% to 25%, or, for example, from 5% to 20%of the weight of the coil pass (7). One of the limiting issues toconsider is the creep of the coil pass (7) given the additional weightof the pins. However, it should be noted that an array of pins willplace less stress on the coil pass than a continuous fin (e.g., there isless mass to support). This may also affect the location andconcentration of the pins. It may reduce creep if there are more pins onthe upper surface of the pass. In some embodiments, the pins have thesame composition as the material of the pass (7) of the radiant coil.

In one embodiment, the tube may be manufactured by welding(electrically) any stud shaped strip (e.g., a wire or a welding rod) tothe surface of the tube at a desired location and then cutting the stripat the desired length.

The pass of the coil may be a tube of stainless steel which may beselected from wrought stainless, austentic stainless steel and HP, HT,HU, HW and HX stainless steel, heat resistant steel, and nickel basedalloys. The coil pass may be a high strength low alloy steel (HSLA);high strength structural steel or ultra high strength steel. Theclassification and composition of such steels are known to those skilledin the art.

In one embodiment, the stainless steel, for example, heat resistantstainless steel may comprises from 13 to 50, or, for example, 20 to 50,or, for example, from 20 to 38 weight % of chromium. The stainless steelmay further comprise from 20 to 50, for example, from 25 to 50, or, forexample, from 25 to 48, or, for example, from about 30 to 45 weight % ofNi. The balance of the stainless steel may be substantially iron.

In some embodiments, the present invention may also be used with nickeland/or cobalt based extreme austentic high temperature alloys (HTAs). Insome embodiments, the alloys comprise a major amount of nickel orcobalt. In some embodiments, the high temperature nickel based alloyscomprise from about 50 to 70, or, for example, from about 55 to 65weight % of Ni; from about 20 to 10 weight % of Cr; from about 20 to 10weight % of Co; and from about 5 to 9 weight % of Fe and the balance oneor more of the trace elements noted below to bring the composition up to100 weight %. In some embodiments, the high temperature cobalt basedalloys comprise from 40 to 65 weight % of Co; from 15 to 20 weight % ofCr; from 20 to 13 weight % of Ni; less than 4 weight % of Fe and thebalance one or more trace elements as set out below and up to 20 weight% of W. The sum of the components adding up to 100 weight %.

In some embodiments, the steel may further comprise a number of traceelements including at least 0.2 weight %, up to 3 weight %, or, forexample, 1.0 weight %, up to 2.5 weight %, or, for example, not morethan 2 weight % of manganese; from 0.3 to 2, or, for example, 0.8 to1.6, or, for example, less than 1.9 weight % of Si; less than 3, or, forexample, less than 2 weight % of titanium, niobium (for example, lessthan 2.0, or, for example, less than 1.5 weight % of niobium) and allother trace metals; and carbon in an amount of less than 2.0 weight %.The trace elements are present in amounts so that the composition of thesteel totals 100 weight %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an ethylene cracker.

FIG. 2 shows the geometry of a single longitudinal vertical fin withrectangular cross section.

FIG. 3 shows an axial fin temperature distribution with increasingheight of the fin.

FIGS. 4A, 4B and 4C show a half cross section of bare tube (FIG. 4A),axial finned tube (FIG. 4B), and pinned tube (FIG. 4C).

FIGS. 5A, 5B and 5C show the outside wall stress distribution for baretube (FIG. 5A), axial finned tube (FIG. 5B), and pinned tube (FIG. 5C).

FIGS. 6A, 6B and 6C show the inside wall stress distribution for baretube (FIG. 6A), axial finned tube (FIG. 6B), and pinned tube (FIG. 6C).

EXAMPLE

The present invention will now be illustrated by the following nonlimiting example.

Example 1

A finite element model of the ethylene 1 furnace tubes was performed inANSYS Mechanical 14.0. This is a commercial finite element analysis(FEA) software used to create numerical models for stress/strain andheat transfer analysis.

Prior to performing a FEA analysis a heat transfer model of arectangular fin (FIG. 2) was created for a one-dimensional heatdistribution. The net heat conducted through the fin is equal to heattransferred to the fin external surface from surroundings,

Q_(x + dx) − Q_(x) = Q_(α):${\frac{d}{dx}( {A\frac{d\mspace{14mu}\Theta}{dx}} )} = {\frac{\alpha\mspace{14mu} O}{\lambda}\Theta}$

Where Θ=t_(g)−t_(x)—the temperature difference between combustion gases,t_(g), and local temperature in the fin, t_(x), at location x (0≤x≤Lz)

i) O=2(L_(s)+L_(h))—the perimeter of the cross section of therectangular fin,

λ—thermal conductivity of the fin material,

α—total heat transfer coefficient (α=α_(rad+conv))

from the above equation.

$\frac{d^{2}\mspace{14mu}\Theta}{{dx}^{2}} = {B^{2}\mspace{14mu}\Theta}$$B \equiv \sqrt{\frac{\alpha\mspace{14mu} O}{\lambda\mspace{14mu} A}}$

The general solution of this equation takes the form:Θ_(x) =C ₁ e ^(Bx) +C ₂ e ^(−Bx)where the constants C₁ and C₂ are determined from two boundaryconditions:

for  x = 0  Θ = Θ_(p) = C₁ + C₂for  x = L_(z)  Θ = Θ_(k) = C₁e^(BL) + C₂e^(−BL)${and},{Q_{\lambda} = {Q_{\alpha} = {{{- A}\;{\lambda( \frac{d\;\Theta}{dx} )}_{x = {Lz}}} = {A\;{\alpha\Theta}_{k}}}}}$

So, after calculating C₁ and C₂, the temperature distribution in the fintakes the form:

$\Theta_{x} = {\Theta_{p} = \frac{{\cosh\lbrack {B( {L_{z} - x} )} \rbrack} + {\frac{\alpha}{B\mspace{14mu}\lambda}{\sinh\lbrack {B( {L_{z} - x} )} \rbrack}}}{{\cosh\mspace{14mu}{BL}_{z}} + {\frac{\alpha}{B\mspace{14mu}\lambda}\sinh\mspace{14mu}{BL}_{z}}}}$

This temperature distribution is shown in FIG. 3 for a base temperatureof 900° C. which was used for generating temperature loads on the axialfinned tube.

A static structural FEA was performed on three different furnace tubes;a bare tube, an axial finned tube, and a pinned tube. Half models werecreated with symmetric boundary conditions. A cross section of each ofthe tubes is shown in FIGS. 4A, 4B and 4C. The temperature distributiondescribed above was applied to the external surface of the finned andpinned tube. Since the above heat transfer analysis was not performedfor a pinned tube, the external surfaces of the pinned tube were assumedto follow the same distribution. An average process temperature ofapproximately 750° C. and an average convective heat transfercoefficient of 998 W/m²K were used to define the thermal boundarycondition on the inner surface of the tube. Both gravity and an internaltube pressure of 0.336 MPa were also applied to the furnace tube model.The temperature distribution described above was determined for an axialfinned tube and the assumption was made that the distribution would besimilar in a pinned tube.

External and internal stress distributions are shown in FIGS. 5A, 5B and5C and FIGS. 6A, 6B and 6C. As seen in these figures, the finned furnacetube is in a much higher state of stress than the bare furnace tube. Thedifference in thermal expansion of the tip and base of the axial fincauses the base tube to be put in a high state of tension.

The advantage of the pinned tube is that it is not constrained in anydirection and is free to expand. There is a slight stress concentrationat the base of the pin; however the overall state of stress is muchlower than that of the axial finned tube. The overall state of stress inthe furnace tube is comparable to that of a bare tube. However, there isan increase in heat transfer in the pinned tube over the bare tube.

What is claimed is:
 1. A tube for use in the radiant section of afurnace for cracking hydrocarbons to produce olefins having on itsexterior surface a series of pins in one or more linear arrays parallelto the longitudinal axis of the tube, said pins having: i) a maximumheight from 0.8 to 1 cm; ii) a contact surface with the tube, having anarea from 0.1% to 10% of the tube external surface area; iii) a uniformcross section along the length of the pin; iv) length to diameter ratiofrom 1.5:1 to 0.5:1; v) a distance between consecutive pins within agiven linear array is from 0.1 to 5 times the diameter of the pin; vi)from 2 to 6 linear arrays of pins radially spaced from 180° to 20°apart; and vii) the aggregate weight of the pins comprises from 5 wt. %to 25 wt. % of the weight of the tube.
 2. The tube according to claim 1,wherein the pins have a maximum length from 3% to 15% of the tube outerdiameter.
 3. The tube according to claim 2 comprising from about 55 to65 weight % of Ni; from about 20 to 10 weight % of Cr; from about 20 to10 weight % of Co; and from about 5 to 9 weight % of Fe and the balanceone or more of the trace elements.
 4. The tube according to claim 3further comprising from 0.2 up to 3 weight % of Mn; from 0.3 to 2 weight% of Si; less than 5 weight % of titanium, niobium and all other tracemetals; and carbon in an amount of less than 0.75 weight % the sum ofthe components adding up to 100 weight %.
 5. The tube according to claim2, comprising from 40 to 65 weight % of Co; from 15 to 20 weight % ofCr; from 20 to 13 weight % of Ni; less than 4 weight % of Fe and thebalance of one or more trace elements and up to 20 weight % of W the sumof the components adding up to 100 weight %.
 6. The tube according toclaim 5, further comprising from 0.2 up to 3 weight % of Mn; from 0.3 to2 weight % of Si; less than 5 weight % of titanium, niobium and allother trace metals; and carbon in an amount of less than 0.75 weight %.7. The tube according to claim 2, comprising from 20 to 38 weight % ofchromium from 25 to 48, weight % of Ni.
 8. The tube according to claim7, further comprising from 0.2 up to 3 weight % of Mn, from 0.3 to 2weight % of Si; less than 5 weight % of titanium, niobium and all othertrace metals; and carbon in an amount of less than 0.75 weight % and thebalance substantially iron.
 9. The tube according to claim 3, whereinthe cross section of the pin is round.
 10. The tube according to claim5, wherein the cross section of the pin is round.
 11. The tube accordingto claim 7, wherein the cross section of the pin is round.
 12. The tubeaccording to claim 4, wherein the cross section of the pin is round. 13.The tube according to claim 3, wherein the cross section of the pin isquadrilateral.
 14. The tube according to claim 5, wherein the crosssection of the pin is quadrilateral.
 15. The tube according to claim 7,wherein the cross section of the pin is quadrilateral.
 16. The tubeaccording to claim 4, wherein the cross section of the pin isquadrilateral.
 17. Tube according to claim 2, wherein the pins in alinear array are of uniform height.
 18. The tube according to claim 2,where in the spacing between pins in a linear array is from 0.5 to 5times the diameter of the pin.
 19. The tube according to claim 2,wherein the pins in a linear array are of different heights to provide aprofile to the array.
 20. The tube according to claim 19, wherein atleast part of the profile is a taper.
 21. The tube according to claim 2,where in the central axis of the pin is at an angle from 90° to 60°relative to the external surface of the tube.
 22. A method for making atube according to claim 1, electrically welding a strip of any studshaped material to the surface of the tube and then cutting the strip atthe desired length.